1. Field of the Invention
The present invention relates to a variable magnification projection optical system capable of changing a variable magnification ratio to project a magnified image light formed in an image forming element onto a screen and an image projection apparatus using the variable magnification projection optical system.
2. Description of the Related Art
An image projection apparatus which is generally called a projector is an apparatus which projects a magnified image light formed in an image forming element on a screen. In particular, with respect to image projection apparatuses for cinema, with the spread of digital cinema, miniaturization and high definition of image forming elements are required, and accordingly, miniaturization and high resolution of variable magnification projection optical systems installed in the image projection apparatuses for cinema are also required.
As these variable magnification projection optical systems, there are optical systems disclosed in JP 2001-108900 A, JP 2003-015038 A, JP 2002-350727 A, and JP 2008-052174 A. The projection zoom lens disclosed in JP 2001-108900 A is a projection zoom lens which magnifies a plane image and projects and images the magnified plane image. First to sixth groups having respective negative, positive, positive, negative, positive, and positive refracting powers are arranged in order from the magnification side. During changing of a projection distance, in order to conjugate the plane image with a projection surface, the first group is moved in an optical axis direction, and during changing of magnification, the first group, the fourth group, the sixth group are fixed, and the second group, the third group, and the fifth group are moved in the optical axis direction. A focal length fw of the entire system at the wide-angle end, a back focus bf (value in air), a total lens length OAL (length from the surface of the first group closest to the magnification side to the surface of the sixth group closest to the reduction side), a focal length f3 of the third group, an average value ν6P of Abbe numbers of convex lenses constituting the sixth group, an average value ν4M of Abbe numbers of concave lenses constituting the fourth group, and an average value ν5M of Abbe numbers of concave lenses constituting the fifth group satisfy the condition expressions: OAL>90·bf/fw, 1.5<f3/fw<2.5, ν6P>50, and (ν4M+ν5M)/2<40.
The projection zoom lens disclosed in JP 2003-015038 A is a projection zoom lens where, first to sixth groups having the respective refracting powers of negative, positive, positive, negative, positive, and positive are arranged in order from the magnification side, and an aperture stop is arranged between the third and fourth lens groups. During continuously changing of magnification from the wide-angle end to the telephoto end, the first, fourth, and sixth lens groups are fixed, and the second, third, and fifth lens groups are moved on the optical axis. A focal length fw of the entire system at the wide-angle end, a focal length f1 of the first lens group, a back focus Bf when the conjugate point of the magnification side is infinity, and a length L of the entire system satisfy the conditions: 1.4<Bf/fw, 1.0<|f1|/fw<1.7, and 6.5<L/fw<9.0.
In addition, the projection zoom lens disclosed in JP 2002-350727 A is a projection zoom lens where six components of first to sixth groups having respective negative, positive, positive, negative, positive, and positive refracting powers are arranged in order from the magnification side, and the projection zoom lens is substantially telecentric at the reduction side. During change of magnification from the telephoto end to the wide-angle end, the third group and the fifth group are moved from the magnification side to the reduction side, and a stop is arranged in the fourth group.
In addition, the zoom lens disclosed in JP 2008-052174 A, is a zoom lens which magnifies and projects display elements and is configured to include, in order from the magnification side, six groups of first to sixth groups. The zoom lens is configured to include, a first group having a negative refracting power, a second group having a positive refracting power and being moved during changing of magnification, a fifth group having a positive refracting power and being moved during changing of magnification, and a sixth group having a positive refracting power and to include a third group having a positive or negative refracting power and being moved during changing of magnification and a fourth group having a negative refracting power which includes a stop or a third group having a negative refracting power which includes a stop and a fourth group having a positive or negative refracting power and being moved during changing of magnification. The sixth group is configured to include one positive lens, and when a distance on an optical axis from a center of a lens surface at the reduction side of the positive lens of the sixth group to a surface of a display element arranged perpendicular to the optical axis is denoted by Bf, when a radius of curvature of the positive lens of the sixth group at the magnification side is denoted by CR1, and when a minimum F value of the zoom lens is denoted by F, a condition expression 1.35≦Bf/CR1×F≦2.00 is satisfied. In the case where the third group has a negative refracting power and includes a stop and the fourth group has a positive or negative refracting power and is moved during changing of magnification, at least one of the positive lenses constituting the fourth group and the fifth group is configured with a material satisfying nd>1.58νd>59, and herein a refractive index with respect to d-line (wavelength of 587.56 nm) is denoted by nd and an Abbe number with respect to d-line (wavelength of 587.56 nm) is denoted by νd, and in the case where fourth group has a negative refracting power and includes a stop and the third group has a positive or negative refracting power and is moved during changing of magnification, at least one of the positive lenses constituting the fifth group is configured with a material satisfying nd>1.58νd>59. In addition, in the case where the third group has a negative refracting power and includes a stop and the fourth group has a positive or negative refracting power and is moved during changing of magnification, a composite focal length of the fourth group and the fifth group at the time of the zoom lens being at the wide-angle end is denoted by f×w, and in the case where the fourth group has a negative refracting power and includes a stop and the third group has a positive or negative refracting power and is moved during changing of magnification, a focal length of the fifth group is denoted by f×w. In the case where the zoom lens is at the wide-angle end, a focal length is denoted by fw. In this case, a condition expression 1.70≦f×w/fw≦4.80 is satisfied.
However, in order to achieve high resolution, suppression of various types of aberration such as spherical aberration, image surface curvature, axial chromatic aberration, and magnification chromatic aberration is required. However, as the variable magnification ratio is increased, the various types of aberration are greatly changed between the telephoto end and the wide-angle end, so that there is a circumstance that the aforementioned requirement is not compatible with the high variable magnification ratio. Therefore, the high resolution and the high variable magnification ratio are difficult to be compatible with each other.
JP 2001-108900 A, JP 2003-015038 A, JP 2002-350727 A, and JP 2008-052174 A are reviewed from this point of view. In the zoom lenses disclosed in JP 2001-108900 A and JP 2003-015038 A, it is considered that, if the high resolution is achieved, the configuration of the fourth lens group including a stop becomes inappropriate, so that it is not possible to effectively suppress the spherical aberration. In addition, in the zoom lenses disclosed in JP 2002-350727 A and JP 2008-052174 A, it is considered that, if the high resolution is achieved, the chromatic aberration of the fourth lens group is insufficiently suppressed, so the chromatic aberration at the wide-angle end and the telephoto end are increased.